s390/nmi: avoid using long-displacement facility

__LC_MCESAD is currently 4528 /* offsetof(struct lowcore, mcesad) */
that would require long-displacement facility for lg, which we don't
have on z900.

Fixes: 3037a52f

 ("s390/nmi: do register validation as early as possible")
Signed-off-by: Vasily Gorbik <gor@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>